[okl4-developer] L4 root task info

[uf65ca]

On Thu, Nov 06, 2008 at 03:18:44PM -0800, uf65ca wrote:
> 
> can anyone shed some light on this task?  what does it do? is it a user
> land
> task?
> 

Traditionally the roottask is the address space and the
initial context of execution running in user mode.

Traditionally the roottask is privileged, unlike threads in the system 
threads in the roottask can perform privileged kernel system calls such 
as creating and destroying threads, or perform mapping operations.  
Note: the privilege check for the 3.0 release works differently.


> in a strictly priority based system, what is the right priority for a root
> task? is there is any restriction as how low can it be? does it make sense
> to make
> it one of the lowest priority task?
>  in term of scheduling, is it just another ordinary task? 

By default the initial thread running in the initial address space
starts with the highest priority.  You can make an L4 system
call to lower its priority.  What the priority should be will 
probably depend on how you construct your system.

L4 strictly obeys the priority for threads with different priority
numbers and performs round-robin scheduling within the same priority
number, which includes the initial thread as well as any threads living 
in the initial address space.

	-gl

*****************************

Thanks for your reply, 

regarding the scheduling of roottask, it's different from what I got from
this link:
http://i30www.ira.uka.de/~l4hq/cgi-bin/research/pistachio-faqwiz/faqw.py?req=all#6.4

"...sigma0, sigma1, and the roottask. Since the scheduler of all three
threads is unkown, invoking the ThreadControl system call can fail. Thus,
none of the mentioned scheduling state of these threads can be reliably
set."


also I don't think you can claim "L4 strictly obeys the priority for threads
with different priority
numbers" as long as "direct switch" mechanism is used in sync-IPC.

Can I set  the priority of the roottask at *build* time, via a config file,
to a rather low level (almost lowest level in the system)?  no, I'm not
talking about lowering roottask priority @ runtime thru syscall?   
is there any hidden assumption that roottask has the highest priority when
created?

Thanks



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